1. Field of the Invention
The invention generally relates to electronics, and in particular, to high-speed communications devices.
2. Description of the Related Art
A common technique used to compensate for channel loss in high-speed serializer/deserializer (SERDES) links is known as transmit amplitude pre-emphasis. A channel 104 has a low-pass response. An example of amplitude pre-emphasis is illustrated in FIG. 1A. That technique pre-distorts transmit signal pulses in amplitude levels based on preceding and/or following data bits so as to reduce residual Inter Symbol Interference (ISI) at the Receiver (Rx) input after the channel. In general, amplitude pre-emphasis boosts the high frequency content of the transmit signal pulses to complement high frequency losses in the channel 104. This works in practice if the channel 104 is linear, which is a valid assumption for backplane applications. However, for non-linear channels, amplitude pre-emphasis is not directly applicable.
With reference to FIG. 1A, the frequency spectrum at the output of the transmit buffer 102 shows the boost in high frequency content of the transmit signal generated by amplitude pre-emphasis to counter act against the higher high frequency loss of the channel 104. At the receiver 106, an overall flat response can be obtained, which lowers inter-symbol interference (ISI).
FIG. 1B illustrates that pre-emphasis does not work well for channels 114 that have signal slicers 112, 116. An example of a non-linear channel is a channel in an optical network. For example, optical-to-electrical (O/E) and electrical-to-optical (E/O) converters act as slicers 112, 116. The high frequency boost from amplitude pre-emphasis is effectively removed by the slicer 112 and, in turn, cannot effectively compensate for channel loss and results in larger loss of high frequency content of the signal. Thus, isolated pulses, which have larger high-frequency content compared to other signal bits, are attenuated more and their amplitude is reduced, making isolated pulses the weak link in the transmission channel. This situation is further exacerbated by Pulse Width Shrinkage (PWS), which can occur in O/E and E/O converters. With sufficient loss in the channel 114, isolated pulses are missed by the slicer 116 on the receive side, which results in increased bit errors.